Surface textures, e.g., three-dimensional patterns, may be imparted to sheet or web materials by a process in which a web is coated with a curable coating, a texture is imparted to the curable coating by a replicative surface in the form of an engraved metal roll, and the coating is cured while in contact with the roll by passing radiation through the web. If the web is transparent, the coating may be cured using ultraviolet (UV) radiation, while if it is opaque electron beam radiation is generally used. Substrates (webs) that can be used with this type of process are limited to those which are thin enough to be penetrated by the radiation, and, in the case of opaque substrates, capable of withstanding treatment with electron beam radiation. In some cases, release from the embossed roll is problematic, and/or there may be problems with the coating quality, e.g., air entrapment or streaking.
The sheet or web materials resulting from this process may be used as release sheets or films in processes in which a plastic film or sheet is formed on or against a release sheet and then separated from the release sheet after cooling or curing to set the plastic material. The release sheet provides a surface from which the set plastic material can be readily separated and imparts to the surface of the plastic material the quality of finish of the release surface. For example, a desired textured surface can be provided on the surface of the plastic material by forming the plastic material on or against a release sheet having a textured surface that is the mirror image of the desired textured surface.
One example of such a forming process is “casting,” a process in which a resinous material, such as polyvinyl chloride or polyurethane resin, in a flowable state, is deposited or “cast” onto the release sheet surface, heated, cured and cooled to consolidate the resinous material into a continuous self-supporting film, and stripped from the release sheet. The release sheet is normally provided with a desired surface effect, such as high gloss, texturing or an embossed configuration, and the surface effect is replicated on the cast film.
U.S. Pat. No. 4,289,821 (Gray et al.) and U.S. Pat. No. 4,322,450 (Gray et al.), the disclosures of which are incorporated herein by reference, disclose techniques for producing surface effects in a release coating on a release sheet for use in casting processes. One method disclosed comprises applying a coating of an electron beam radiation curable material to one surface of a web substrate, pressing the coated side of the substrate against a replicative surface (an embossing medium) having the desired surface effect to cause the coating to conform to the replicative surface, irradiating the coating with electron beam radiation to cure the coating, and stripping the substrate from the replicative surface with the cured coating adhered to the substrate. The replicative surface is preferably a metal roll with either a pattern engraved in its surface or a highly polished smooth surface. An important advantage of this technique is that the pattern or finish of the replicative surface is reproduced in the cured coating with essentially 100% fidelity. This technique enables replication of very fine patterns, such as wood grain and leather grain, on the surface of a plastic cast onto the release sheet.